Some current industrial robots are provided with a wrist mechanism having two coaxial bevel gears, and FIGS. 4A to 4C show an essential portion of a conventional robot wrist mechanism for an industrial robot. As shown in FIGS. 4A to 4C, a bevel gear 11' is attached to the extremity of a shaft A for an .alpha.-drive, and a bevel gear 21' is attached to the extremity of an outer shaft A.sub.2 for a .beta.-drive.
A ball bearing B.sub.1 is pressed against the back surface of the inner bevel gear 11', and a ball bearing B.sub.2 is held in place with a bearing nut 15. A protrusion P.sub.0 formed in the inner circumference of the boss of the outer bevel gear 21' extends between the ball bearings B.sub.1 and B.sub.2. A shim 19 is provided between the front shoulder of the protrusion P.sub.0 and the ball bearing B.sub.1.
A ball bearing B.sub.21 is pressed against the back surface 27 of the outer bevel gear 21', and a ball bearing B.sub.22 is held in place with a bearing nut 25. A protrusion P formed in a housing H extends between the ball bearings B.sub.21 and B.sub.22, and the bearing nut 25 engages a threaded portion 26.
When assembling the robot wrist mechanism, the ball bearings supporting the bevel gears are properly preloaded by adjusting the bearing nuts, and appropriate shims are used to adjust the tooth contact and backlash. A backlash adjustment is also necessary when the teeth of the bevel gears are abraded.
The tooth contact adjustment for the outer bevel gear of the bevel gear unit shown in FIGS. 4A to 4C of the conventional robot wrist mechanism requires a relatively easy procedure including the steps of, unfastening bolts S.sub.2, pulling out an outer bevel gear assembly in the direction of the arrow F.sub.1 as shown in FIG. 4B, changing the shim 29 for another shim 29, replacing the outer bevel gear assembly, and refastening the bolts S.sub.2. On the other hand, the tooth contact adjustment for the inner bevel gear requires a relatively difficult procedure, as shown in FIG. 4C, including the steps of, unfastening the bolts S.sub.2, removing the outer bevel gear assembly, removing the bearing nut 15, pulling out the inner bevel gear 11, in the direction of the arrow F.sub.2, putting a new shim 19 on the boss of the inner bevel gear 11' so as to be in contact with the ball bearing B.sub.1, inserting the inner bevel gear 11' in the outer bevel gear 21' so as to press the shim 19 against the ball bearing B.sub.1 by the protrusion P.sub.0, and fastening the bearing nut 15. Once the bearing nut 15 is thus removed, the preloading must be again performed for the ball bearings B.sub.1 and B.sub.2. Thus, the conventional bevel gear unit requires much and tooth contact adjustment when assembling the bevel gear unit or changing the shim.
The present invention provides a compound bevel gear unit having a new construction, to solve the foregoing problems.